1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a laser beam printer, and more particularly to an image forming apparatus capable of detecting a light from a surface of an image bearing member.
2. Related Background Art
In an image forming apparatus utilizing an electrophotographic image forming process, apparatus of various types are recently being proposed and commercialized. FIG. 10 shows a background technology of the present invention and is a schematic cross-sectional view of a multi-color image forming apparatus.
A photosensitive member unit including a first image bearing member integrally includes a charging member 2 and a cleaning member 9 along a periphery of a drum-shaped photosensitive member 1 constituting the first image bearing member.
An exposure apparatus provided above the photosensitive member unit forms an electrostatic latent image based on image data, on the photosensitive member 1 charged to a predetermined potential by the charging member 2.
A developing apparatus 40 develops the electrostatic latent image, formed on the surface of the photosensitive member 1, with a developer thereby obtaining a developed image. In the image forming apparatus shown in FIG. 10, the developing apparatus 40 is constituted of units of four colors (yellow 4Y, magenta 4M, cyan 4C and black 4K) for forming a multi-color image.
An intermediate transfer unit including a second image bearing member includes an intermediate transfer belt 105 constituting a second image bearing member and so provided as to oppose to the photosensitive member 1 in a position downstream of a contact portion with the developing apparatus 40 in the rotating direction of the photosensitive member 1. Along an internal periphery of the intermediate transfer belt 105, there are provided a first transfer member 54 (roller) and plural rollers 51, 52, 53 for supporting the belt 105. Developed images of respective colors, formed on the photosensitive member 1 constituting the first image bearing member, by the developing apparatus 4M, 4C, 4Y, 4K are transferred in succession and in superposition on the intermediate transfer belt 105 constituting the second image bearing member, whereby the belt 105 bears a multi-color image constituting a basis of a final image.
A transfer apparatus 6 is constituted of a roller-shaped elastic member, and transfers the developed image, on the intermediate transfer belt 105 constituting the second image bearing member, onto a transfer material P supplied at a predetermined timing from a sheet feeding unit 12.
A fixing apparatus 8 is constituted of pressure members 83, 84 incorporating heat sources 81, 82 and fixes the developed image, on the transfer material P, to the transfer material P under application of heat and a pressure, thereby obtaining a multi-color image.
After the transfer step to the intermediate transfer belt 105, toner remaining on the photosensitive member 1 is recovered by a cleaning member 9, while toner remaining on the intermediate transfer belt 105 is recovered by a cleaning member 10 in preparation for a next image formation.
As a multi-color image forming apparatus, in addition to the configuration shown in FIG. 10, there is also known an image forming apparatus of so-called tandem system in which four image forming units, each integrally including a first image bearing member and a developing apparatus, are arranged parallel to an intermediate transfer belt constituting a second image bearing member and execute image formations of respective colors substantially simultaneously thereby improving an image forming efficiency.
In any multi-color image forming apparatus, there has been a remarkable improvement in image quality in recent years, and a higher image quality and a higher stability are desired by the user.
A higher image quality means an image output without a deterioration a color reproduction or an image texture, in comparison with an original image. On the other hand, a higher stability means to constantly reproduce a same image quality in the output image under any environment.
Parameters that may deteriorate such high image quality and stability include environmental characteristics and a deterioration in time of the photosensitive member and the developer.
The multi-color image forming apparatus often employs a control for correcting such changes in the photosensitive member and the developer. For example, the apparatus shown in FIG. 10 employs, for such correction control, a control process of forming a predetermined test pattern on the image bearing member, detecting a density of the test pattern with optical detection means 311 and correcting a charging condition, an exposing condition or a developing condition based on a result of such detection.
In case of executing a density detection of a test pattern on the intermediate transfer belt, a high surface glossiness is required for the intermediate transfer belt. This is because the optical detection means adopts, as a detecting principle for detecting a toner density of the test pattern formed on the intermediate transfer belt, a method of detecting a light amount reflected from the intermediate transfer belt corresponding to the toner density, namely by receiving a reflected light with light-receiving means based on a light emitting from light-emitting means, so that a higher surface glossiness is required for the intermediate transfer belt in order to measure the optical intensity of the received light and to exactly and precisely detect the toner density.
As explained above, in detecting the toner density of the test pattern formed on the intermediate transfer belt, a larger reflected light amount from the belt increases a dynamic range, thereby improving a precision of detection.
Thus, in case of executing an optical detection by measuring the optical intensity of the reflected light from the belt surface and controlling image forming conditions such as an image density control based on such detection, a limited amount of the reflected light from the belt results in an inferior detecting precision, leading to a decrease in the density whereby a high image quality cannot be maintained.